In intaglio printing presses, it is commonly known to use a wiping cylinder contacting the plate cylinder carrying the intaglio printing plate or plates as a wiping device for wiping and cleaning the surface of the intaglio printing plate or plates. The purpose of such a wiping cylinder is to simultaneously press the ink deposited onto the printing plates into the engravings and clean the excess ink from the plenum of the printing plates, i.e. the unengraved area of the printing plates outside the engravings.
In order to achieve good printing quality, the wiping cylinder is commonly designed in such a way that its outer surface contacting the printing plates is both physically and chemically resistant, i.e. is adapted to sustain the high contact pressure and friction with the printing plates and can withstand the physical and chemical contact with the ink components and pigments, as well as with the cleaning solutions which are used to clean the surface of the wiping cylinder.
It has already been proposed to provide such a wiping cylinder with an outer layer of resilient synthetic composition, namely a heat-hardenable plastic composition such as PVC. U.S. Pat. No. 3,785,286, U.S. Pat. No. 3,900,595 and U.S. Pat. No. 4,054,685 for instance disclose methods for making such wiping cylinders as well as apparatuses for implementing the said methods. These publications are incorporated by reference in the present application, especially in respect to the material used for forming such cylinders and to the machines and methods used for building such wiping cylinders. Referring for instance to the coating apparatus described in U.S. Pat. No. 4,054,685, means are provided for horizontally mounting a cylinder to be coated for rotation about its axis of rotation. Coating is performed by rotating the cylinder past a straight-edged scraper blade mechanism disposed at one side of the cylinder and which extends parallel to the cylinder axis, this blade mechanism being adapted to be moved towards and away from the cylinder. The blade mechanism consists of two blades mechanically coupled to each other, namely a lower blade and an upper blade which are jointly designed to ensure a proper supply of heat-hardenable plastic material to the surface of the cylinder to be coated and allow adjustment of the thickness of the material to be deposited. The blade mechanism is adapted to be moved towards and away from the cylinder while maintaining the straight edge of the lower blade (i.e. the edge which extends along the length of the cylinder) parallel to the axis of rotation of the cylinder. The plastic material is supplied to the blade mechanism on top of the upper blade which is disposed, during coating of the cylinder, in an inclined relationship with respect to the cylinder so as to form a reservoir between the upper side of the upper blade and the periphery of the cylinder to be coated. Means are provided for restraining flow of the plastic material sideways from the reservoir. The blade mechanism can be translated towards and away from the cylinder in order to maintain a desired uniform spacing (a couple of millimeters or less) between the straight edge of the lower blade and the periphery of the cylinder along the full length of the cylinder. The cylinder is rotated in a direction to cause its periphery to move downwardly past the blade mechanism to thereby apply to the periphery of the cylinder a thin uniform layer of plastic composition having a thickness determined by the spacing between the straight edge of the lower blade and the periphery of the cylinder. This layer of plastic material is heat-cured by applying radiant heat to the cylinder throughout its length as the cylinder is rotated so as to cause hardening of the deposited layer of plastic material and produce a hardened layer of the desired hardness. Several layers with different hardnesses and thicknesses are preferably formed in this way onto the cylinder surface.
According to the solutions described in U.S. Pat. No. 4,054,685, supply of the plastic material to the surface of the cylinder is either interrupted by removing the upper blade of the blade mechanism or by retracting the upper blade away from the cylinder, the upper blade sliding on top of the lower blade.
U.S. Pat. No. 5,180,612 discloses another type of apparatus for coating a wiping cylinder with a layer of plastic material which, in contrast to the previous apparatuses, makes use of a twin-roller coating unit for the application of the plastic material onto the surface of the cylinder. Such a solution has a number of disadvantages including in particular the higher complexity of the coating unit as well as it greater size which affects the ability of the operator to efficiently monitor the coating process and take corrective measures during the coating process. Further, this solution requires an additional cooling unit to regulate the temperature of the rotating coating rollers and prevent undesired hardening of the plastic composition before it reaches the surface of the cylinder. Lastly, cleaning of the coating unit at the end of the coating process is made much more complicated due to the inherently complex nature of the coating unit with its two rotating rollers.